The present invention relates generally to methods and apparatus for deviated drilling of wells, such as oil and gas wells, and more specifically relates to improved whipstocks and methods of their use to initiate a deviated wellbore at a desired depth and in a desired direction relative to an existing wellbore.
Whipstocks have been utilized for many years to drill deviated boreholes "kicking-off" from an existing wellbore. These deviated boreholes may extend from a so-called "vertical" or so-called "horizontal" wellbore. As is well-known, virtually no borehole is truly "vertical" and therefore virtually all wellbores have a low side and a high side. Typically, whipstocks include a ramp inclined from one side of the wellbore to the other. This ramp is adapted to guide a mill toward the sidewall of the casing to enable a section of the casing and the surrounding formation to be removed to initiate the deviated borehole.
Conventional whipstocks have been subject to a significant limitation in that conventional whipstocks have typically required their orientation with the ramp extending along the low side of a borehole. This orientation has been necessary with conventional designs to assure that the ramp of the whipstock would not become displaced into the center of the borehole where it would both fail to serve the necessary guiding function and would also be subject to significant damage from attempts to operate the mill within the wellbore above the whipstock.
This need for orientation has several adverse consequences. First, the necessary orientation of conventional whipstocks on the low side of the wellbore causes the milling to be performed on the high side of the wellbore. In highly deviated wellbores, this results in the milling being done in opposition to the force of gravity. Additionally, it would only be a matter of circumstance for any given drilling operation if the optimal kick-off location for a deviated borehole was actually on the high side of the wellbore. Thus, the limitations of conventional whipstocks have often required either compromises in the orientation of the well or the use of expensive navigational drilling equipment to direct the deviated borehole to its desired end location, after kicking off on the high side of the borehole. As a further consequence, the orientation of whipstocks has required the use of surveying and orienting equipment to be certain that the whipstock was orientated with the ramp along the low side of the borehole, to avoid the problems identified above. Further, the required orientation of conventional whipstocks has required that the whipstocks be run into the wellbore on a work string which could be rotated to achieve the necessary orientation along the low side of the borehole. This requirement has effectively precluded the use of coil tubing to set whipstocks.
Another drawback with conventional retrievable whipstock mechanisms has been the difficulty of retrieving whipstocks from the well after use. Most conventional retrieval mechanisms have required the use of a "female" sleeve which would slip over at least a significant portion of the whipstock guide section to mechanically engage the guide section and to remove the whipstock from the well. Such sleeve type apparatus while effective, can be relatively expensive to construct, and relatively awkward to utilize.
Accordingly, the present invention provides a new whipstock and method of use, wherein the whipstock does not require orientation relative to the borehole, and therefore may be run into the borehole in any desired manner, including on coil tubing; but which may be oriented in any desired relation to the wellbore, without concern for orientation along the low side of the wellbore. The present invention further provides an improved whipstock and retrieval mechanism facilitating the removal of the retrievable whipstock from a wellbore to a relatively simple and inexpensive cooperative arrangement between the whipstock and a retrieval tool.